1. Field of the Invention
The present invention relates to a method of cooling on a cooling bed hot-rolled steel sections from rolling heat, wherein the sections, particularly rails, have section parts of different masses arranged at a distance from each other over the cross-section of the section; the present invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
Rails are cooled on cooling beds from the rolling heat to temperatures below 80xc2x0 C. Because of the asymmetrical arrangement of the masses of the section, different cooling behaviors occur between the head and the base of the rail, wherein the base cools more quickly than the head because the base has larger heat radiation surfaces in relation to the mass thereof. As a result, the rail buckles during cooling. This buckling can be counteracted to a certain extent by pre-bending the rail while it is still in the hot state. However, this procedure has the disadvantage that it requires a complicated hot bending step whose outcome is uncertain. In any event, the rails must be realigned after cooling. The cooling process as well as especially the alignment produce stresses in the rail which disadvantageously influence the strength of the rail. Numerous proposals have become known for effectively counteracting this difficulty.
DE 42 37 991 A1 describes a method for cooling profiled rolling stock, particularly rails, which are hot-rolled in rolling stands, wherein cooling is effected by natural convection or with forced air cooling. The gist of the known method resides in that the rails are transported over the cooling bed with the head hanging downwardly. This measure favorably influences the heat transition conditions already with natural convection in such a way that the temperature difference between head and base of the rail is reduced from about 140xc2x0 C. when the head of the rail faces up to about 50xc2x0 C. with the rail hanging facing down. Because of the small temperature difference between head and base, the disadvantages of buckling are effectively reduced and an almost straight rail can be introduced into the straightening machine for final straightening, so that the final stresses in the rail material can be kept extremely low.
German Patent 21 61 704 describes a method and an apparatus for cooling railroad rails in a stress-free and distbrtion-free manner. In accordance with this method, similar rail sections to be cooled are clamped together in pairs base against base, so that the rail bases are arranged next to each other and form abutments for each other, wherein the rails clamped together in this manner are conveyed over a cooling bed by a transverse conveyor. Since each rail head has approximately the same mass as the rail base while the circumference of the rail base is approximately twice that of the rail head, the ratio of the circumferential surface of the clamped-together rail bases relative to their combined mass is about the same as the ratio of the circumferential surface and the mass of the rail head. This results in a uniform cooling of rail heads and rail bases; it has been found in practice that this measure of rail bases resting in pairs against each other is sufficient for an almost distortion-free cooling.
U.S. Pat. No. 468,788 discloses a method for cooling rails in which the rails are immersed entirely or partially in a device with downwardly directed rail heads in a basin filled with water and are cooled in this manner, wherein the rails are simultaneously pressed against a fixed abutment by means of pressure screws.
German Patent 404 127 discloses a method for aligning metal rods having asymmetrical cross-sections, particularly railroad rails, wherein the thicker parts of the cross-section are subjected to a controlled artificial cooling in such a way that all parts shrink by the same extent in spite of the unequal thicknesses and the rods remain straight when cooled down to ambient temperature. This result is achieved by producing the artificial cooling either by immersing the rails in a liquid, by wetting or sprinkling, by blowing a dispersed liquid, by means of steam, air or another gas, wherein the medium used is continuously or intermittently applied during the duration or only during a portion of the cooling period. A significant aspect of this method is the fact that the artificial cooling can be controlled in such a way that the rods are not hardened during cooling even when they are composed of high-carbon steel or a hardenable alloy.
German patent 19 42 929 discloses a method for cooling rails which is based on a different physical principle. In this method, prior to reaching the austenite transformation temperature, the rails are placed at a distance above a heat-reflecting layer on the rail base. In addition, during the further course of cooling, a solid insulation material may be placed on the running surfaces of the rails. A mutual positive influence by radiation is further achieved in this method by placing the rails immediately next to one another, so that the rail bases touch each other at the sides thereof. These measures result in a positive influence on the cooling pattern of each part of the rail cross-section without the supply of outside energy because of reflection at a reflection layer and an insulating cover of the running surfaces. This creates an advantageous stress compensation in the rail cross-section. Placing the rails on the rail base prior to reaching the austenite transformation temperature at a distance above a heat-reflecting layer produces the advantage that an early start of the austenite transformation in the rail base and the rail web are prevented. Consequently, the technological values of the rail material can be influenced individually, i.e., depending on steel analysis, by an exact temperature adjustment in such a way that higher strength values, expansion values and constriction values can be achieved.
The opposite result, i.e., a hardening of the rail head due to an appropriate rapid cooling procedure, is achieved in accordance with French patent 543,461 by subjecting the rail hanging downwardly with the rail base facing up to a series of defined immersion procedures of very short duration in a trough filled with water.
All the above-described methods have the disadvantage that they are based more or less on empirical results, i.e., it is first necessary to carry out extensive tests to determine the parameters that must be maintained while carrying out the method in order to achieve the desired cooling result. These tests require that at least for each charge test pieces of hot-rolled sections are used and that the tests must be repeated when the results are not immediately satisfactory, so that waste material is produced initially in many cases.
Therefore, it is the primary object of the present invention to improve a method and an apparatus of the above-described type for cooling hot-rolled sections and to perfect the method and the apparatus to such an extent that the above-described difficulties are overcome and a distortion-free cooling result is achieved when cooling from tolling heat without requiring expensive and extensive tests and without producing moist material.
In accordance with the present invention, initially the heat quantities to be proportionally removed from the different section parts in dependence on their mass and temperature and the quantity of cooling medium required for this removal are determined and computed by using measuring equipment together with a computing unit by means of a computer program. Subsequently, cooling of the different section parts or their masses is carried out in a controlled manner in such a way that the section parts reach with as little time delay as possible the transformation line Ar3/Ar1 during the decomposition of the gamma-mixed crystal into ferrite and/or pearlite while releasing the transformation heat.
In accordance with another feature of the invention, the steel section is subjected with its head facing downwardly and its head facing upwardly to a series of defined immersion procedures of very short duration in a trough filled with water.
The method according to the present invention produces the advantageous result that excellent cooling is achieved in different charges without curvature of the section and without requiring expensive empirical experiments.
A further development of the present invention provides that the further cooling from the transformation temperature to a predetermined final temperature is carried out in such a way that the different mass centers of the section reach the final temperature with as little time delay as possible. In addition to an excellent cooling result without curvature of the rail, this measure ensures an optimum temper condition with uniform hardness over the entire cross-section of the section.
In accordance with a useful feature, when the quantities of heat to be removed from the section parts are computed, the transformation temperature of the basic steel quality is taken into consideration.
Another advantageous further development of the invention provides that, when the rolled section or parts thereof are cooled by means of water as the cooling agent, the heat transfer numbers occurring at the different section surfaces are determined and these heat transfer numbers are used for predetermining the quantities of cooling agents required for cooling the section surfaces. This eliminates extensive experiments and lost test material.
In accordance with another advantageous embodiment of the present invention, the rails are guided over a cooling bed with the rail heads facing downwardly and a controlled cooling of the different mass centers is carried out at least partially by natural convection and additionally by the additional use of cooling medium based on the heat quantities to be removed proportionally in dependence on the mass and temperature of the section parts. This reduces a curvature of the section to such an extent that a realignment of the section is either not necessary at all, or only a slight realignment is required which does not produce harmful stresses.
The heat removal may be carried out by directing, preferably intermittently, a spray of cooling medium against the individual section parts.
In order to compensate a temperature wedge which may exist over the length of the section, another feature of the present invention provides that the rolled section is cooled with different intensity over the rolled length.
Finally, a controlled heat removal can also be carried out by immersing the entire rolled section or individual parts thereof in a cooling medium either once or repeated several times with predetermined cycle periods.
An apparatus for carrying out the method according to the present invention includes means for measuring the heat radiation of different section parts, such as, head, web or base. These measuring means are arranged next to a rail preferably spaced apart from each other along the rolling length. The measuring means are connected through data lines to a computing unit in which the dimensions or masses of these section parts are entered with an input data line, and which is programmed in such a way it computes the product of temperature and mass and controls the cooling medium discharge device through a signal line in dependence on this product.
In accordance with a preferred feature of the apparatus, the cooling bed includes controllable cooling medium discharge devices for different cooling media, for example, water, air, water/air mixtures.